In water, we get faultless service (5)
I believe the answer is:
'in water' is the definition.
The definition suggests a singular noun which matches the answer.
'we get faultless service' is the wordplay.
'get' says to put letters next to each other.
'faultless service' becomes 'ser' (I can't justify this - if you can you should give a lot more credence to this answer).
Can you help me to learn more?
(Other definitions for weser that I've seen before include "German runner" , "Er, sew up the German river" .)